1. Field of the Invention
The present invention relates to an exposure apparatus which exposes a substrate, and a device manufacturing method using the same.
2. Description of the Related Art
A projection exposure method used in manufacturing, for example, semiconductor devices has been known. In this method, a pattern formed on an original (reticle) is projected onto a substrate coated with a photoresist by a projection optical system to expose the substrate.
Japanese Patent Laid-Open No. 10-161295 proposes an interference exposure method using a diffraction grating (phase-shift grating) as one exposure method of forming a fine pattern with a simple arrangement. The interference exposure using a diffraction grating is an exposure method of forming an interference pattern on a substrate by a plurality of diffracted light beams generated by irradiating a diffraction grating with a light beam having coherency, thereby exposing the substrate using this interference pattern. This exposure method uses an arrangement simpler than that used in the projection exposure method, and therefore has the feature that it can easily form a periodic interference pattern with a high resolution and a large depth of focus at a low apparatus cost.
As another interference exposure method using a diffraction grating, Japanese Patent Laid-Open No. 2007-027350 proposes a method of scanning and exposing a large-area exposure region by scanning a substrate with respect to a diffraction grating. In this method, two diffraction gratings having different grating pitches are used so that light beams diffracted at nearly the same point on the first grating are diffracted by the second grating and superposed on each other again at nearly the same point on the substrate. Hence, a high-contrast interference pattern can be obtained even when the gratings are irradiated with low-coherency illumination light.
When high-coherency light is used as the exposure light, an unwanted interference pattern called a speckle is generated. The term speckle refers to a speckled intensity distribution generated as high-coherency light (coherent light) is scattered by the minute three-dimensional pattern on the object plane, and the scattered light beams interfere with each other on the substrate in a random phase relationship. The contrast of a finally obtained pattern may decrease as speckle-induced noise components are superposed on a targeted interference pattern, resulting in local defects of the final pattern.